Page 84 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 61

point (called cavitation). Any vaporization should be
avoided to ensure proper operation of the meter and
to retain the relationship between pressure difference
and flow. Vaporization can be prevented by main-
taining the inlet pressure sufficiently high and the
inlet temperature sufficiently low.
. What is an orifice meter?
& An orificemeter consistsofa circularplate,containing
a hole (orifice), which is inserted into a pipe such that
the orifice is concentric with the pipe inside diameter.
. What is the principle of operation of orifice meter?
& As the fluid approaches the orifice, the pressure
increases slightly and then drops suddenly as the
orifice is passed. It continues to drop until vena
contracta is reached and then gradually increases
until at approximately 5–8 diameters downstream a
FIGURE 4.3 Vena contracta.
maximum pressure point is reached that will be lower
than the pressure upstream of the orifice. The de-
isflowingoutatwhichthepressurebecomesthelowest
crease in pressure as the fluid passes through the
with fluid velocity attaining its maximum value.
orifice is a result of the increased velocity of the fluid
& Vena contracta is the point in a fluid stream where the
passing through the reduced area of the orifice. When
diameter of the stream is the least (Figure 4.3).
the velocity decreases as the fluid leaves the orifice
& The maximum contraction takes place at a section
the pressure increases and tends to return to its
slightly on the downstream side of the orifice, where
original level. All of the pressure loss is not recovered
the jet is more or less horizontal.
because of friction and turbulence losses in the
& The reason for this phenomenon is that fluid stream-
stream. The pressure drop, DP, across the orifice
increases when the rate of flow increases. lines cannot abruptly change direction. In the case of
both the free jet and the sudden pipe diameter change,
& With the orifice plate or any flow measurement based
the streamlines are unable to closely follow the sharp
on differential pressure, pressure drop is at zero at no
angle in the pipe/tank wall. The converging stream-
flow and then increases with the square of the flow.
lines follow a smooth path, which results in the
Thus, if the application requiring a 5-to-1 turndown
narrowing of the jet (or primary pipe flow) observed.
in flow, a differential pressure transmitter with a
turndown of 25-to-1 is required. This can create . What is sharp-edged orifice? What is its advantage?
limitations with the application of orifice meters. & With sharp-edged orifice, the cross section of dis-
& Similarly, when sizing an orifice to handle the max- charging liquid, contracts as it leaves the opening.
imum flow rate, it may not be possible to also Figure 4.4 illustrates a sharp-edged orifice along with
measure the flow at the lower end of the range, due square and rounded orifices. Sharp-edged orifices are
to loss of signal. used as standard orifices. Flow through the other two
orifices is affected by the thickness of the plate and
& Pressure drop for a gas flow is based on the flow rate at
the roughness of the surface, and for the rounded
the gas density at the actual operating conditions. To
type, the radius of curvature.
getamass flowmeasurementatstandardconditionsof
temperature and pressure, it is also necessary to have & Velocity attainable will be maximum and conse-
temperature and pressure transmitters and a flow quently pressure will reach its lowest value at vena
computer or multivariable transmitter. As a result, contracta and pressure differential developed will be
while the cost of the orifice plate itself is relatively highest, making accuracy of measurements by a
inexpensive, the installed cost of the complete system manometer more precise.
becomes substantially more expensive due to the & If the orifice is blunt edged, for a given flow rate, head
additional instrumentation that is required to obtain will be less than that for sharp-edged orifice.
an accurate mass flow measurement. . For a sharp-edged orifice, how is the area of vena
. What is vena contracta? contracta related to downstream pressure?
& Vena contracta is the position on the downstream & Vena contracta area increases with decrease in down-
side of an orifice plate or a nozzle from which a fluid stream pressure.